1. Field of the Invention
The present invention relates to a method of and an apparatus for expanding the mode field diameter of an optical fiber by heating the optical fiber with a burner to thermally diffuse the dopant that forms the refractive-index profile.
2. Description of the Background Art
In recent years, researchers and engineers have been developing an optical fiber transmission line in which an optical fiber having a small mode field diameter, such as an optical fiber for wavelength division multiplexing transmission or an optical fiber for Raman amplification, is coupled with an ordinary single-mode optical fiber having a relatively large mode field diameter. When optical fibers having different mode-field diameters are spliced, it is difficult to reduce the splice loss to a practical level only by a simple fusion-splicing method. To solve this problem, Japanese patent No. 2618500 has disclosed a method in which a fusion-spliced portion is additionally heated to thermally diffuse the dopant in the core portion toward the cladding portion. This method can produce a fusion-spliced portion in which the mode field diameter of at least one optical fiber is gradually varied so that the optical fibers can be spliced with the same mode field diameter. Hereinafter, this fusion-spliced portion is referred to as a thermally-diffused expanded core (TEC).
FIGS. 12A1, 12A2, 12B1, and 12B2 are diagrams showing examples for forming a TEC. FIGS. 12A1 and 12A2 are diagrams showing an example in which a TEC is formed after two optical fibers having different mode field diameters are fusion-spliced. FIGS. 12B1 and 12B2 are diagrams showing an example in which a TEC is formed in an optical fiber having a smaller mode field diameter before the optical fibers are fusion-spliced.
In these figures, optical fibers 1a and 1b to be fusion-spliced have cladding portions 2 having the same diameter and core portions 3a and 3b having different mode field diameters and different relative refractive-index differences. The optical fibers 1a and 1b have resin jackets 4. The optical fibers 1aand 1b are butt-fusion-spliced by fusing the mutually facing ends of the optical fibers by using arc discharge or another means. If they are spliced only by a simple fusion-splicing method, the optical fibers 1a and 1b cannot be spliced with the same mode field diameter, and therefore the splice loss increases.
To solve this problem, as shown in FIG. 12A1, a TEC is formed by additionally heating the fusion-spliced portion 5 including its neighboring portions with a microtorch or burner 6 fed with fuel gas. The heating is performed under temperature and time conditions that thermally diffuse the dopant, which is added into the core portions 3a and 3b to increase the refractive index, toward the cladding portions 2 without fusing the optical fibers 1a and 1b. As shown in FIG. 12A2, the heating expands the mode field diameters to form an expanded portion 7 in which the mode fields can be smoothly spliced.
The optical fiber 1a having a smaller mode field diameter and a higher dopant concentration allows the dopant to thermally diffuse more than the dopant in the optical fiber 1b having a larger mode field diameter and a lower dopant concentration. Consequently, the mode field diameter in the optical fiber 1a is expanded more than that in the optical fiber 1b, reducing the discrepancy between the two mode field diameters. As explained above, it is known that when different types of optical fibers are fusion-spliced, the splice loss can be reduced by forming a TEC in which a smaller mode field diameter is expanded such that it closely approximates the mode field diameter of the other optical fiber.
In the case of the other example, as shown in FIG. 12B1, first, the central portion of an optical fiber 1a having a smaller mode field diameter is heated to expand the mode field diameter so that an expanded portion 7 can be formed. Second, the expanded portion 7 is cut to obtain a splicing end face 5′ having the same mode field diameter as that of an optical fiber 1b to be spliced. Under this condition, the optical fibers are spliced as shown in FIG. 12B2. This example, also, can prevent an increase in splice loss resulting from the unsmooth splicing caused by a discrepancy in mode field diameter. The published Japanese patent application Tokukaishou 61-117508 has disclosed that the formation of such a TEC is effective even for splicing optical fibers having the same design feature, because the TEC can reduce the splice loss caused by core eccentricity.
The foregoing TEC is formed usually by using a microtorch or a burner. A specified region of an optical fiber is heated by giving the microtorch longitudinal movements relative to the optical fiber. Alternatively, a plurality of microtorches or burners may be placed along the optical fiber. The published Japanese patent application Tokukaihei 8-82721 has disclosed another method in which a plurality of optical fibers arranged in a flat array are concurrently heated by using a plurality of microtorches or burners arranged in the arrayed direction of the optical fibers. Tokukaihei 8-82721 has also disclosed a method in which a fiber ribbon is heated by using a microtorch specifically designed to correspond to the width of the fiber ribbon.
It is necessary to form a TEC by heating the optical fiber under proper temperature and time conditions. The optical fibers 1a and 1b are heated at a temperature below their melting points. Nevertheless, if the heating is not properly conducted, the heated portion is softened and may bend due to the weight of the optical fiber itself. If bending occurs and remains, it increases the splice loss. In addition, it is difficult to control the heating condition when a burner is used, because the flame of a burner has a specific temperature distribution and wanders according to the variation in environmental condition.
Sometimes, a plurality of TECs are formed at fusion-spliced portions after the individual fibers of a fiber ribbon comprising 8, 12, 24, or more fibers are collectively fusion-spliced. In this case, the flames of the burner heat the fusion-spliced portions such that the flames engulf the multiple optical fibers arranged in a flat array. Consequently, the outer fibers in the array are more intensely heated than the inner fibers. In other words, the optical fibers in the array are not uniformly heated. As a result, the TECs are not uniformly formed. This creates a problem in that the splice losses in the individual optical fibers are not uniformly reduced.
The above-described phenomena in the formation of a TEC or TECs are significantly affected by the structure of the heating burner used. However, conventional burners have difficulties in controlling the conditions for properly heating a specified region of an optical fiber, particularly an optical fiber incorporated in a fiber ribbon.